In cellular wireless communications large cells are often sectored to increase cell capacity. Typically, three 120.degree. sector antennas are used to cover one cell and a receiver is switched between the three sector antennas.
Another application which involves switching a receiver between two or more antennas is space diversity which is used to reduce the effect of multipath fading in radio receivers. By using two antennas placed an adequate distance apart, a receiver can be made such that it selects the one with the strongest signal, thereby giving an overall improvement in reception.
FIG. 1(a) shows the basic concept in which a switch selectively connects a low noise amplifier (LNA) forming part of the receiver to one or other of two antennas. This arrangement, however, is generally not practical since the switch will have some insertion loss which will degrade the receiver's sensitivity to weak signals. FIG. 1(b) shows the common topology with the switch moved farther down the receiver chain for greater sensitivity. In this case two LNAs are connected between the respective antennas and the switch such that, in effect, the switch selects the output of one or other of the LNA's. A problem with this arrangement is that, as both LNA's are running continuously, it requires twice the current and is more complicated because it requires additional output switch circuitry.
The problem of high current requirement is overcome in a device marketed under the part number TQ9203 by Triquint Semiconductor, Inc. as described in the Triquint data sheet entitled "Low-Current RFIC Downconverter" and dated Apr. 21, 1994. This device is a multifunction RF downconverter in which an LNA section comprises two parallel connected common source FET (field effect transistor) amplifiers each having an input connected to a respective antenna and a common output. A "Select" or "Control" terminal controls a bias circuit connected directly to the gates of the two transistors such that, when a control signal indicative of a stronger reception signal on one antenna is applied, the transistor connected to the one antenna is switched on and the other transistor is switched off and, when the control signal has a value indicative of a stronger signal on the other antenna, the transistors are switched to the opposite state. In this way the stronger antenna signal is conducted to the common output.
One problem with the Triquint device is that it has a relatively high gain variation with temperature. In addition it is designed only for operation in the range 800 to 1000 MHz and would not work at low frequency because of AC coupled inputs.
A device which overcomes the above-mentioned disadvantages of the prior art devices is disclosed in U.S. patent application Ser. No. 08/822,460 filed on Mar. 21, 1997 in the name of Tiller et al and assigned to the assignee of the present application. The disclosure of U.S. patent application Ser. No. 08/822,460 is incorporated herein by reference.
In a particular embodiment described in the Tiller et al application three antennas are respectively connected to three amplifying transistors which have a common output and three control inputs are connected respectively to three switching transistors. When a particular one of the control inputs receives a control voltage indicating that the associated antenna is to be selected, the respective switching transistors turns on such that current flows in the corresponding amplifying transistor. This circuit uses for each amplifying transistor a bias feedback circuit which includes resistors in series with the switching transistor.
A potential drawback of the Tiller et al device is that those antennas which are not selected are simply disabled and are not terminated into a fixed resistance. The effect of this is that a poor return loss is associated with the disabled antenna inputs. Because radio base station transceivers are required to be terminated to 50 ohms this means that the Tiller et al device cannot be used in base stations having such requirements without modification.